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man pages section 3: Basic Library Functions

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Updated: Thursday, June 13, 2019
 
 

gets(3C)

Name

gets, fgets - get a string from a stream

gets_s - get a string from a stream with additional safety checks

Synopsis

#include <stdio.h>

char *gets(char *s);
char *fgets(char *s, int n, FILE *stream);
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>

char *gets_s(char *s, rsize_t n);

Description

The gets() function reads bytes from the standard input stream (see intro(3)), stdin, into the array pointed to by s, until a newline character is read or an end-of-file condition is encountered. The newline character is discarded and the string is terminated with a null byte.

If the length of an input line exceeds the size of s, indeterminate behavior may result. For this reason, it is strongly recommended that gets() be avoided in favor of fgets().

The fgets() function reads bytes from the stream into the array pointed to by s, until n−1 bytes are read, or a newline character is read and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null byte.

The fgets() and gets() functions may mark the st_atime field of the file associated with stream for update. The st_atime field will be marked for update by the first successful execution of fgetc(3C), fgets(), fread(3C), fscanf(3C), getc(3C), getchar(3C), getdelim(3C), getline(3C), gets(), or scanf(3C) using stream that returns data not supplied by a prior call to ungetc(3C) or ungetwc(3C).

The gets_s() function is part of the bounds checking interfaces specified in the C11 standard, Annex K. It is similar to the gets() function, but with additional safety checks provided via explicit runtime-constraints as defined in the C11 standard. See runtime_constraint_handler(3C) and INCITS/ISO/IEC 9899:2011.

Return Values

Upon success, the gets() and fgets() functions return s. If end-of-file is encountered and no bytes have been read, no bytes are transferred to s and a null pointer is returned. For standard-conforming (see standards(7)) applications, if the end-of-file indicator for the stream is set, no bytes are transferred to s and a null pointer is returned whether or not the stream is at end-of-file. If a read error occurs, such as trying to use these functions on a file that has not been opened for reading, a null pointer is returned and the error indicator for the stream is set. If end-of-file is encountered, the EOF indicator for the stream is set. Otherwise s is returned.

Upon success, the gets_s() function returns s. If there is a runtime-constraint violation, or if end-of-file is encountered and no bytes have been read into the array, or if a read error occurs during the operation, a null pointer is returned.

Errors

Refer to fgetc(3C).

The function gets_s() will fail if:

EINVAL

Null pointer is passed.

ERANGE

Size argument is not a valid value.

Usage

The fgets() function allows properly-written programs to safely process input lines too long to store in the result array. In general, it requires that callers of fgets() pay attention to the presence or absence of a new-line character in the result array. Consider using the fgets() function (along with any needed processing based on new-line characters) instead of the gets_s() function.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
See below
Standard

The gets() and fgets() functions can be used safely in multithreaded applications.

The gets_s() function cannot be used safely in a multithreaded application due to the runtime constraint handler. For more information, see the runtime_constraint_handler(3C) man page.

Note

The use of gets() function is discouraged since the user cannot specify the length of the buffer passed to gets(). The length of the string read is unlimited. It is possible to overflow this buffer in such a way as to cause applications to fail, or possible system security violations.

Applications should use the fgets() function instead of the obsolescent gets() function.

The ISO C99 standard Technical Corrigendum 3 (ISO/IEC 9899:1999 Cor. 3:2007(E)), the X/Open Portability Guide Issue 7 (XPG7), and the IEEE POSIX 1003.1-2008 standard have all declared the gets() function is obsolescent and deprecated, and may be removed in future versions.

The ISO C11 standard (ISO/IEC 9899:2011) does not include gets(). When compiling with __STDC_VERSION__ >= 201112L, which specifies C11 mode, then the <stdio.h> header will not provide a function prototype for gets().

See Also

lseek(2), read(2), ferror(3C), fgetc(3C), fgetwc(3C), fopen(3C), fopen_s(3C), fread(3C), getchar(3C), getdelim(3C), getline(3C), scanf(3C), scanf_s(3C), stdio(3C), ungetc(3C), ungetwc(3C), attributes(7), standards(7), runtime_constraint_handler(3C)